Pituitary adenylate cyclase activating polypeptides (PACAP) are potent cardiovascular regulatory neuropeptides. We hypothesize that a critical site of PACAP action is within the peripheral parasympathetic cardiac ganglia that serve as local integrative centers. In guinea pig parasympathetic cardiac ganglia, all postganglionic neurons are innervated by PACAP-immunoreactive fibers, and PACAP peptides potently depolarize and increase excitability of the postganglionic cardiac neurons. The increase in membrane excitability of guinea pig cardiac parasympathetic neurons by PACAP is greater than that produced by any other neuropeptide studied to date, but mechanism(s) responsible for the PACAP-induced depolarization and increase in excitability are not known. The primary goal of the project is to establish the mechanisms underlying the PACAP-induced increase in excitability and depolarization of guinea pig cardiac neurons. The following specific aims are proposed. Aim 1: Establish the second messenger transduction cascades responsible for generation of the PACAP-induced increase in membrane excitability and depolarization. Aim 2: Establish the membrane ionic mechanisms underlying the PACAP-induced increase in membrane excitability. Aim 3: Establish the membrane ionic mechanisms generating the PACAP-induced inward current. Aim 4: Establish whether the PACAP-induced rise in intracellular calcium (Ca2+)i in cardiac neurons occurs from either Ca2+ influx or from Ca2+ release from IP3- or ryanodine-sensitive stores, and test whether the rise in (Ca2+)i is an initial step in the generation of the membrane depolarization or increased excitability. The results of these experiments will provide key, new insight into mechanisms determining PACAP-induced excitatory actions on neurons in the parasympathetic cardiac ganglia; ganglia that determine the extent of parasympathetic inhibitory drive on cardiac function.